It consisted mainly of Austro-Hungarian coastal bombardments of Italy′s Adriatic coast, and wider-ranging German–Hungarian submarine warfare into the Mediterranean.

Allied forces mainly limited themselves to blockading the German–Hungarian navies in the Adriatic, which was successful in regards to surface units, but failed for the U-boats, which found safe harbours and easy passage into and out of the area for the whole of the war. Considered a relatively secondary part of the naval warfare of World War I, it nonetheless tied down significant forces.

One day after the French declaration of war against Austria-Hungary on 11 August, the French fleet—under Admiral Augustin Boué de Lapeyrère—entered Malta. He had orders to sail with all available French and British ships, pass into the Adriatic Sea and undertake whatever operation he thought best against an Austrian port. Lapeyrère decided to surprise the Austrian vessels enforcing a blockade of Montenegro. The main Allied force comprised the French battleships Courbet, Jean Bart, and the cruiser Jurien de la Gravière. Two French squadrons of pre-dreadnoughts, two squadrons of cruisers, and five destroyer squadrons were held back in support. The British support group comprised two armored cruisers and three destroyer divisions. The Anglo-French force succeeded in cutting off and sinking the old Austro-Hungarian light cruiser SMS Zenta off Bar on 16 August in Battle of Antivari.

Throughout most of late August most of the action was simple bombardment of Serbian and Montenegrin troops by Austrian ships. On 9 August, the pre-dreadnought SMS Monarch shelled the French radio station at Budva, while the destroyer SMS Panther shelled Mount Lovcen. On 17 August, Monarch shelled a Montenegrin radio station off Bar, then another station off Volovica Point on 19 August. Meanwhile, a French squadron shelled Austrian troops on Prevlaka.

The French and Montenegrin forces attempted to cause havoc also at Cattaro in September, October and November 1914, and the KuK navy was called in there also, resulting in a decisive defeat for the Allies.

Both the French and the Austrians spent much of this time laying extensive minefields throughout the shallow waters of the Adriatic. Mostly this was done by destroyers, and at night. Several steamships ran afoul of these mines and either sunk or were damaged.

On 7 August, an Austro-Hungarian Fleet—consisting of six battleships, two cruisers, and 19 destroyers and torpedo boats—sortied from Pola to escort Goeben and Breslau through Austro-Hungarian territorial waters, returned to port following day without ever making contact. Goeben and Breslau briefly engaged HMS Gloucester and the chase was abandoned by the British. By 10 August, both German warships were safely in the Dardanelles and heading for Turkey.

In November, the French submarine Cugnot managed to slip into the Bocche di Cattaro as far as Topla Bay but was chased out by the Austrian destroyer SMS Blitz, and the torpedo boat Tb 57T. Later that month, the French submarine Curie raided the harbor barrage of Pola to wait for her chance to intrude. Two days later, on 20 December, during an attempt to sneak into the harbor she got entangled in an anti-submarine net and could not free herself. Forced to surface for fresh air, she was sunk by the Austrian destroyer SMS Magnet and Tb 63T, with three casualties. The Austrians raised the wreck between December 1914 and February 1915. It was then repaired and commissioned as U-14 in June 1915.

On 21 December, the submarine U-12 scored one torpedo hit on the French battleship Jean Bart off Sazan Island. The battleship had to withdraw to Malta for extensive repairs.

In February, the French destroyer Dague—while escorting the transport Whitehead to Bar—was sunk after hitting a mine. Also that month, the Austrian submarine U-12 was unsuccessfully attacked off Cape Mendra by a French submarine. Austrian destroyer SMS Csikós shelled Montenegrin positions at Bar with Tb 15 and Tb 68F.

The destroyer SMS Velebit shelled the Italian airship Città di Ferrara off Ancona. The pre-dreadnought SMS Radetzky and two torpedo boats bombarded Potenza Picena, then returned to Pola. The Radetzky-class pre-dreadnought SMS Zrínyi, with two torpedo boats bombarded Senigallia, destroying a train and damaging a railway station and a bridge, then returned to Pola. The torpedo boat Tb 3 was unsuccessfully bombed by an Italian airship. The light cruiser SMS Admiral Spaun shelled the Italian signal station at Cretaccio Island, while SMS Sankt Georg—with two torpedo boats—shelled Rimini, damaging a freight train. The destroyer SMS Streiter shelled the signal station near Torre di Mileto. The light cruiser SMS Novara, a destroyer and two torpedo boats entered Corsini Channel and shelled an Italian torpedo boat station, a semaphore station, and coastal artillery batteries.

The light cruiser SMS Helgoland—aided by four destroyers—sank the Italian destroyer Turbine in a pitched battle south of Pelagosa. The destroyer SMS Tátra shelled the railway embankment near Manfredonia while the destroyer SMS Csepel shelled the Manfredonia railway station.

In response, on 5 June, four different Allied task forces attacked the Austrian coast. Four Italian armoured cruisers, escorted by four French destroyers, shelled Cavtat; the British cruiser HMS Dublin—escorted by five Italian destroyers—shelled Donzella; the Italian light cruiser Quarto—escorted by four destroyers—bombarded Lastovo; the Italian light cruiser Nino Bixio, two Italian and two French destroyers shelled the island of Lissa. On 9 June, a mixed force of British, French and Italian destroyers shelled the Austro-Hungarian signal station at Cape Rondini in Albania.

The armored cruiser SMS Sankt Georg and a squadron of torpedo boats bombard Rimini on 16 June, causing minor damage. Then on 17 June, the cruisers SMS Novara, Admiral Spaun and their escorts attacked and sank the Italian steamer Maria Grazia off Giulianova. The next day, they shelled Rimini and Fano, destroying the Italian signal station there.

The summer of 1915 was a successful time for Austrian submarines as well: on 10 June, U-11 sank the Italian submarine Medusa and torpedo boat Serpente; U-10 sank the Italian torpedo boat PN 5 on June 26 off Venice; U-4 torpedoed and sank the Giuseppe Garibaldi on 18 July; and U-5 captured the Greek steamer Cefalonia off Durazzo on August 29. But this was not without losses. On 13 August, U-3 was sunk at Brindisi by the French destroyer Bisson, after having been severely damaged by the Italian auxiliary cruiser Città di Catania the day before.

The Austro-Hungarian naval air-arm also began regular bombing raids against Bari and Brindisi in June, slightly damaging the British protected cruiser HMS Amethyst in one such raid with machine gun fire. And the British armed trawlerSchiehallion was sunk by a mine. The Amalfi was sunk off Venice by the German submarine UB-14 on July 2. While the Italian scout cruiser Marsala shelled Gravosa station on 18 July, the scout cruiser Quarto and three Italian destroyers attacked the Austrian installation at Guiparra.

SMS Helgoland, seven destroyers and four torpedo boats supported an Austrian landing at Pelagosa on 28 July. The landing was repulsed by the Italian garrison, that had arrived in the island on July 11. On August 17 the light cruisers "Helgoland", "Saida" and several destroyers bombarded the island again. The water reservoir was severely damaged and the next day the Italians began the evacuation. On 17 August, one of the cruisers was unsuccessfully torpedoed by an Italian submarine on return journey. The last act of the summer was the sinking on 26 September of the Italian battleship Benedetto Brin in Brindisi Harbor by Italian-speaking Austro-Hungarian saboteurs. Over 450 were killed.

In early December, the French submarine Fresnel ran aground off the Bojana River estuary due to bad navigation, and was sunk by the Austrian destroyer SMS Warasdiner. The cruiser SMS Helgoland and three destroyers sortie against the Otranto Barrage from 5–22 December and performed reconnaissance off the Albanian coast and San Giovanni di Medua. They sank an Italian picket boat, three steamships loaded with ammunition and two armed schooners en route to Northern Albania.

The light cruiser SMS "Helgoland " and five Tatra-class destroyers left Cattaro and headed for Durazzo late on 28 December 1915. While on passage the French submarine Monge was rammed by the cruiser SMS Helgoland, and finally sunk by gunfire from the destroyer SMS Balaton. Early the next day, the Austrians squadron opened fire on Durazzo targets, sinking some small ships. Then they ran into a minefield. The destroyer "Lika" was sunk and "Triglav" was damaged. She was taken in tow and the Austrian force sailed slowly to north. An allied force had already sailed from Brindisi, with the aim to intercept them. It was composed by the British light cruisers HMS "Dartmouth" and "Weymouth", the Italian light cruisers RN "Quarto" and "Nino Bixio" and five French destroyers. The Austrians despatched from Cattaro in support of the retreating force the armoured cruiser SMS "Kaiser Karl VI" and the light cruiser "Novara". Early in the afternoon of 29 December, the forward Allied ships came into action with the retreating Austrian light squadron which was still only halfway home. The "Triglav" was abandoned and scuttled and a long-range gunnery duel was fought throughout the afternoon. SMS "Tatra" was damaged but the Austrian light force was able to reach Cattaro safely.

Austrian submarines sank or damaged a number of ships in 1916. U-11 captured the Italian hospital ship King Albert on 18 January at San Giovanni di Medua. U-6 sank the French destroyer Renaudin on 16 March at Durazzo. On 8 June, U-5 torpedoed and sank the Italian troop transport Principe Umberto at Linguetta. Later, U-5 fought a French-Italian destroyer group to a stalemate on 2 August, and torpedoed the Italian Q-Ship Pantelleria south of Taranto on August 14.

On 15 September 1916, the two Austro-Hungarian seaplanes L.132 and L.135 forced the French submarine Foucault to surface by dropping bombs. L.135 finally sinks the sub while the 27 survivors were clinging to the two planes now floating, to be finally saved by the alarmed Tb 100M. This was the first sinking of a submarine by airplanes in naval war history.

The very same day, the French submarine Ampére scored two torpedo hits on the Austro-Hungarian Hospital ship No I (the former Lloyd steamer Elektra) off Cape Planka (Rat Ploca), causing two fatalities. The damaged hospital ship had to be beached in Borovica Bay for further repairs.

On the night of 22/23 December, the Austro-Hungarian destroyers SMS Scharfschuetze, Reka, Dinara and Velebit attacked the drifters patrolling the Otranto barrage, which applied for help to the French destroyers Casque, Protet, Commandant Rivière, Commandant Bory, Dehorter and Boutefeu which were escorting a convoy from Brindisi to Taranto. Because of communication problems, only Casque and Commandant Rivière attacked, but Casque '​s boiler rooms were hit immediately and she had to slow down to 23 kn (26 mph; 43 km/h). For further assistance, the Italian destroyers Giuseppe Cesare Abba, Ippolito Nievo and Rosolino Pilo left Brindisi shortly followed by the British cruiser HMS Gloucester escorted by Impavido and Irriquieto. The French and Italian groups met during darkness, Giuseppe Cesare Abba rammed Casque; some moments later, Boutefeu rammed Giuseppe Cesare Abba. While the damaged vessels had to be taken into tow, the Austrians escaped in the darkness.

The return from the Otranto battle—15 May 1917—brought the British cruiser HMS Dartmouth within the range of the UC-25 which had already laid mines off Brindisi.

At 13:30, UC-25 torpedoed Dartmouth approximately 36 mi (31 nmi; 58 km) off Brindisi, for some time the ship was considered to be lost, but was manned by a rescue crew later and finally towed into port. On hearing that Dartmouth had been torpedoed, Boutefeu went to assist, only to hit one of UC-25 '​s mines.

By August 1917, Lt. Von Trapp and U-14 had sunk more than 24,000 long tons (24,000 t) of enemy shipping, including the Italian steamer Milazzo (11,480 long tons (11,660 t)). U-4 torpedoed the French steamer Italia near Taranto on 30 May, and on 16 November U-43 severely damaged the Italian steamer Oriona between Brindisi and Valona. On the night of 9–10 December, while SMS "Wien" and "Budapest" were at anchor in Trieste, two Italian MAS (motor torpedo boats) managed to penetrate the harbor defenses undetected and fired several torpedoes at the two ships. "Wien" was struck by two torpedoes and sank in less than five minutes with the loss of 46 of her crew.

At 03:30 on the morning of 10 June 1918, the battleship SMS Szent István—in the company of SMS Tegetthoff and seven other ships en route to attack the Otranto Barrage—was hit by two torpedoes launched from the ItalianMAS-15 Motor Torpedo Boat under Corvette Captain Luigi Rizzo (who had already sunk SMS "Wien") near Premuda island, near Zara. Many of the 1,087 crew were asleep, getting rested for the battle expected in a few hours. Immediate chaos soon changed into frantic efforts to save the vessel which was rapidly shipping water. Even Tegetthoff was hit by another torpedo from a second MAS, but it did not explode.

The last moments of SMS Szent István, hit by a torpedo of the Italian MAS of Luigi Rizzo.

Then Tegetthoff—which had at first sped away from the vicinity of the torpedo attack—returned and took Szent István in tow, in an attempt to reach the massive dry dock at Pula. However, the pumps were unequal to the task before them and the ship continued to slowly list, sinking at 06:12, almost 3 hours after being hit.[1]

It is said that faults in the Tegetthoff-class design: relatively low displacement and high centre of gravity, together with the tremendous weight of twelve 305 mm (12 in) main artillery, did not assist in the salvage operation. There were, however, only 89 dead, partly attributed to the fact that all sailors with the K.u.K. had to learn to swim before entering active service. The attack on the Otranto Barrage was cancelled as a consequence of this attack.[1]

On 2 October, an allied fleet composed of Italian, British, Australian and American warships attacked the port of Durazzo, Albania during the Second Battle of Durazzo. The fleet consisted of over 55 vessels along with MAS boats and supporting aircraft. Allied forces destroyed Austro-Hungarian shore batteries and defeated a small squadron of patrol craft while sustaining comparatively light damage. Durazzo was left in flames, several building, bridges and railroad targets were bombarded which forced the evacuation of the city. A week or so after the battle an allied army occupied the city without resistance.

Many Austro-Hungarian and German U-boats operated out of the Adriatic for the whole of the war. Due to lack of cooperation of the Allies in the Mediterranean control zones, and the late institution of the convoy system, U-boats experienced substantial success throughout the first war years.[2]